1. Field of the Invention
The present invention relates to a metal nanocomposite powder reinforced with carbon nanotubes and to a process of producing a metal nanocomposite powder homogeneously reinforced with carbon nanotubes in a metal matrix powder.
2. Related Art
In order to fabricate carbon nanotube-reinforced composites, many studies have focused on a powder—powder blending between carbon nanotubes and raw metal or ceramic powder. For example, B. Q. Wei (Wei, B. Q. et al., Carbon 37:855–858 (1999)), and S. R. Dong (Dong, S. R. et al., Materials Science and Engineering, A313:83–87 (2001)), suggest an aluminum or copper matrix composite material reinforced with carbon nanotubes, which is synthesized using a powder mixing process and a conventional sintering process. However, characterization of these carbon nanotube-reinforced composite materials show low enhancement, or even a decrease, of mechanical properties. In particular, the relative density of the sintered composite materials is very low, ranging from 85% to 95%. The relative density of composites is important since low relative density means the existence of many fracture sources, such as pores and defects, which can originate from low mechanical properties. There are two reasons for these problems. One comes from the severe agglomeration of carbon nanotubes in a metal matrix. The other is the use of conventional consolidation processes. The present invention mainly focuses on a solution to prevent agglomeration of carbon nanotubes in a metal matrix. In order to homogeneously disperse carbon nanotubes in a metal matrix, metal nanocomposite powders homogeneously reinforced with carbon nanotubes are fabricated. Even though the conventional process for producing carbon nanotube reinforced metal matrix composites contains a fabrication process, e.g. ball milling, for homogeneous blending of carbon nanotubes and metal powder, this process is not an effective method of dispersing carbon nanotubes. Therefore, the present invention proposes a new metal nanocomposite powder homogeneously reinforced with carbon nanotube in a metal matrix and a new method for producing these powders.
Previously, the blending of the carbon nanotubes and metal or ceramic powders did not yield satisfactory results. This is due to difficulty in homogeneously dispersing carbon nanotubes in a metal matrix by simple ball milling processes. Composite powder fabricated by conventional processes show severe agglomeration of carbon nanotubes, especially on the surface of metal powder, and the nanotubes are not homogeneously dispersed inside the metal matrix. The agglomeration of carbon nanotubes on the surface of the metal powder prevents metal matrix powder from being sintered during consolidation, since aggregates on the surface of metal powder can interrupt the diffusion pathway of metal atoms between metal powders. This leads not only to low sinterability of matrix materials including the carbon nanotubes but also to low relative density of composite materials. That is to say, after the consolidation process, the agglomerated carbon nanotubes become pores in the composite material.
FIGS. 1A and 1B contain scanning electron micrograph (SEM) pictures showing the surface structure of a conventional composite powder. These figures demonstrate that a simple mixing and dispersing process, such as a ball milling process, does not prevent the carbon nanotubes from being agglomerated and does not provide a uniform mixing of carbon nanotubes with the matrix powder. In other words, it is impossible to produce a sound metal composite powder or material through a conventional mixing or dispersing process. The term “sound” as used herein means that the carbon nanotubes are not agglomerated on a surface of the metal powder, but rather are homogeneously dispersed in the metal powder.